The invention relates generally to the field of coating a liquid composition onto the surface of a moving web, and, more particularly, to a method for coating a shear-thinning liquid composition onto the surface of a moving web while using an electrostatic field to assist the coating operation.
In all liquid coating systems, there is an upper speed limit, or critical speed, for coating at which the boundary layer of air carried on the substrate surface to be coated is no longer squeezed out at the coating point but rather becomes entrained under the impinging liquid composition. This is typically referred to as air entrainment. Air entrainment can disrupt the uniform application of composition to the web substrate and can result in unacceptable uniformity of coating. Air entrainment is a gross failure and can occur in all methods for coating moving web. Air entrainment occurs predominantly at high coating speeds.
It is known to those skilled in the art that an electrostatic force of attraction between the coating liquid and the surface of the web to be coated can be used to increase coating speed. This is typically referred to as a coating operation with an electrostatic assist. With an electrostatic assist the web and/or the coating apparatus is electrostatically charged to generate an electrostatic force of attraction between the coating liquid and the surface of the web to be coated. For example, a dielectric web carrying a net voltage bias on the surface can exhibit increased apparent wettability and a consequent increase in acceptable coating speed when conveyed around a grounded coating roller. Means for applying such a charge to a web ahead of the coating point are disclosed, for example, in European Patent Nos. EP 0 390 774 B1 and EP 0 530 752 B1 and U.S. Pat. Nos. 4,835,004; 5,122,386; and 5,295,039.
In bead coating operations there are at least three other coating gross failures that may be encountered in addition to Air Entrainment. xe2x80x9cBreaklinesxe2x80x9d (also known as low flow limit) are a running phenomenon in which a uniform, stable bead cannot be maintained, and the bead degenerates into an array of individual cells with gaps in between. This results in a crossweb array of regions of heavy coating interspersed with regions of no coating. xe2x80x9cPull-throughxe2x80x9d is a running phenomenon in which a portion of the composition in the bead is stripped from the underside of the bead by the stabilizing suction or vacuum pressure and is pulled down the suction drain, resulting in varying areas of thin or blotchy coating. Broad, irregularly-spaced streaks may appear in the coating at vacuum pressure levels below the vacuum pressure at which a portion of the coating composition is pulled down the drain. These streaks, which mark the onset of pull-through (also known as weeping or bleeding), are often labeled as Pull-Through, but they are also known in the art as xe2x80x9cHigh Suction Streaksxe2x80x9d (referred to herein as HSS). xe2x80x9cRakelinesxe2x80x9d (also known as ribbing) are a running phenomenon in which the bead assumes a regular array of alternately thick and thin areas, typically between 200 to 400 cycles per meter, resulting in a crossweb array of areas of heavy coating interspersed with areas of light coating. As observed in the process of discovering this invention, the application of electrostatic assist, while decreasing coating tendency toward Air Entrainment and Breaklines, can actually make some coatings more susceptible to Rakelines.
If the coating parameters are such that Breaklines and/or Air Entrainment may occur, then they will occur at relatively low levels of suction (vacuum pressure) and will often occur simultaneously. If the coating parameters are such that Pull-Through and/or Rakelines may occur, then they will occur at relatively high levels of suction (vacuum pressure) and may occur simultaneously. We define the xe2x80x9cCoating Windowxe2x80x9d in terms of the range of vacuum pressure level at which xe2x80x9cacceptable coatingxe2x80x9d can be performed. The term xe2x80x9cacceptable coatingxe2x80x9d as used herein is intended to mean coating free of any and all gross failures defined above. The lower boundary of a coating window is the coating vacuum pressure level below which Breaklines or Air Entrainment occur. The upper boundary of a coating window is the coating vacuum pressure level above which Rakelines, high suction streaks, or Pull-Through occur. The xe2x80x9ccoating windowxe2x80x9d represents a commonly accepted measure of bead coating performance.
xe2x80x9cSuction Noise Sensitivityxe2x80x9d (referred to herein as Sensitivity) is the degree to which a given amplitude of suction variation or noise will modulate the thickness of the coating in the direction of the moving web. Sensitivity is also a common measure of bead coating performance, but it is less important than the Coating Window because the degree of non-uniformity induced by suction noise is usually less significant than the non-uniformity caused whenever a gross failure occurs. Sensitivity is dependent on many coating parameters, particularly the thickness of the layer(s), especially the bottom layer thickness, where a thinner bottom layer is more sensitive. Therefore, the uniformity requirements of the bottom layer may limit the thickness of the bottom layer to be greater than some minimum thickness.
It is known in the art that increasing the viscosity of compositions for coating can improve coating uniformity by increasing resistance to layer deformation by air currents both on the hopper slide and after application to a substrate. Preferably, for gelatin-based compositions, an increase in viscosity is achieved by reducing the amount of water in the composition. However, if the coating thickness is at a minimum due to sensitivity constraints, then increasing the amount of gelatin in the composition will increase the viscosity, but it is desirable to keep the gelatin fraction low (typically less than about 4%) to avoid premature reaction with other ingredients, like crosslinking agents. It is also known, however, that the bottom layer in a bead coating process, whether a single layer or the bottom layer of a multiple-layer pack, must exhibit a relatively low apparent viscosity, i.e. less than 10 centipoises and preferably less than 5 centipoises, at the point of dynamic wetting where the liquid composition first contacts the substrate surface. These requirements, that is, high viscosity under low shear conditions and low viscosity under high shear conditions, as well keeping the gelatin fraction low, may be met by formulating the bottom composition to be pseudoplastic, or non-Newtonian, by including in it an amount of a shear-thinning thickening agent. See, for example, U.S. Pat. No. 4,113,903 issued Sep. 12, 1978 to Choinski, the relevant disclosure of which is hereby incorporated by reference. Such agents are well known in the art of coating compositions, and may include, but are not limited to, sodium cellulose sulfate and other salts of cellulose; copolymers of methyl vinyl ether and maleic anhydride; salts of polyvinyl hydrogen phthalate; polystyrene sulfuric acid; sodium poly(styrenesulfonate); and sulfonated vinyltoluene polymers. It is known in the art that removing gelatin and adding shear-thinning thickening agents can increase the maximum coating speed permissible without Air Entrainment. However, chemical incompatibilities with some layer ingredients may prevent the use of those shear-thinning thickening agents that meet the preferred requirement of 5 centipoises at 10,000 secxe2x88x921, thus limiting the speed permissible without Air Entrainment and increasing the potential for Breaklines.
It is therefore an object of the present invention to provide an improved coating method whereby coatings having improved thickness uniformity may be made.
It is a further object of the invention to provide an improved coating method whereby the coating speed for uniform coatings may be increased.
It is a still further object of the invention to provide an improved coating method whereby the size of the Coating Window is increased.
It is a still further object of the invention to provide an improved coating method whereby the rheological constraints on formulation of the bottom layer are relaxed.
It is a still further object of the invention to provide an improved coating method whereby the total wet thickness for uniform coatings may be reduced.
It is a still further object of the invention to provide an improved coating method whereby the wet thickness of the bottom layer, which usually contains an excess of water for uniform coatings, may be reduced.
It is a still further object of the invention to provide an improved coating method whereby an increased tendency of a coating composition to form Rakelines in the presence of electrostatic coating assist is reduced.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by employing shear-thinning thickening agents in the bottom layer in a bead coating process thereby yielding a pseudoplastic bottom or carrier layer. Simultaneously, an electrostatic field is generated at the coating point to provide an electrostatic force of attraction between the coating liquid and the surface of the web to be coated. The combination of these two independent method steps unexpectedly provides uniform coatings at higher speeds and with greater coating stability than can be achieved with either step practiced independently.
More particularly, in a first step of the method of the present invention, the liquid composition is formulated as a pseudoplastic liquid having a viscosity of at least about 8 centipoises at a shear rate of 100 secxe2x88x921 and a viscosity below 10 centipoises at a shear rate of 100,000 secxe2x88x921, and preferably between 8 and 200 centipoises at a shear rate of 100 secxe2x88x921 and a viscosity below 5 centipoises at a shear rate of 10,000 secxe2x88x921. One class of Theological fluids meeting these requirements have a consistency m greater than 50 and a flow behavior index n less than 0.7 and a viscosity substantially given by
xcex7=m(dxcex3/dxcfx84)nxe2x88x921xe2x80x83xe2x80x83(Eq. 1)
where xcex7 is viscosity and dxcex3/dxcfx84 is the shear rate. The liquid composition may comprise a single-layer coating or may be the bottom composition of a plurality of superposed compositions forming a multiple-composition coating pack for forming a multiple-layer coating. Then, in a subsequent step of the present invention, an electrostatic field is applied between the coating bead of liquid composition and the web surface to be coated such that the coating bead is attracted to the surface, forming a uniform coated layer of the composition thereupon.
The method of the present invention increases the range of permissible coating parameters, such as speed, wet thickness, and viscosity, without incurring any gross failures, while also increasing the resistance of coating to otherwise inherent coating process non-uniformities.